Hydro-generator plant

ABSTRACT

The magnetic circuit of a generator in a hydro-generator plant is arranged to directly supply a high supply voltage of 20-800 kV, preferably higher than 36 kV. The generator is provided with solid insulation and its winding includes a cable ( 6 ) comprising one or more current-carrying conductors ( 31 ) with a number of strands ( 36 ) surrounded by at least one outer and one inner semiconducting layer ( 34, 32 ) and intermediate insulating layers ( 33 ). The outer semiconducting layer ( 34 ) is at earth potential. The stator winding may be produced with full or fractional slot winding, the phases of the winding being Y-connected. The Y-point may be insulated and protected from over-voltage by means of surge arresters, or else the Y-point may be earthed via a suppression filter. The invention also relates to a hydro-generator plant, a generator included in the plant and a procedure for building such a plant.

TECHNICAL FIELD

The present invention relates to a hydro-generator plant of the typedescribed in the preamble to the claim and which is intended forconnection to distribution or transmission networks, hereinafter calledpower networks. The invention also relates to an electric generator forhigh voltage in a hydro-generator plant intended for the above-mentionedpurpose. The invention further relates to a procedure for assemblingsuch a plant and the manufacture of such a generator.

BACKGROUND ART

The magnetic circuits in electric generators usually comprise alaminated core, e.g. of sheet steel with a welded construction. Toprovide ventilation and cooling the core is often divided into stackswith radial and/or axial ventilation ducts. For larger machines thelaminations are punched out in segments which are attached to the frameof the machine, the laminated core being held together by pressurefingers and pressure rings. The winding of the magnetic circuit isdisposed in slots in the core, the slots generally having a crosssection in the shape of a rectangle or trapezium.

In multi-phase electric generators the windings are made as eithersingle or double layer windings. With single layer windings there isonly one coil side per slot, whereas with double layer windings thereare two coil sides per slot. By coil side is meant one or moreconductors combined vertically or horizontally and provided with acommon coil insulation, i.e. an insulation designed to withstand therated voltage of the generator to earth.

Double-layer windings are generally made as diamond windings whereassingle layer windings in the present context can be made as diamond orflat windings. Only one (possibly two) coil width exists in diamondwindings whereas flat windings are made as concentric windings, i.e.with widely varying coil width. By coil width is meant the distance inarc dimension between two coil sides pertaining to the same coil.

Normally all large machines are made with double-layer winding and coilsof the same size. Each coil is placed with one side in one layer and theother side in the other layer. This means that all coils cross eachother in the coil end. If there are more than two layers these crossingscomplicate the winding work and the coil end is less satisfactory.

It is considered that coils for rotating generators can be manufacturedwith good results within a voltage range of 3-20 kV.

It is also generally known that connection of a synchronous machine to apower network must be via a Δ/Y-connected or step-up transformer, sincethe voltage of the power network is generally higher than the voltage ithas hitherto been able to achieve with the electric machine. Thus thistransformer and the synchronous machine constitute integrated parts of aplant. The transformer entails an extra cost and also has the drawbackthat the total efficiency of the system is reduced. If, therefore, itwere possible to manufacture electric generators for considerably highervoltages, the step-up transformer could be eliminated.

Although the dominant known technology for supplying current from agenerator to a high-voltage network, a concept which in the presentapplication applies to the level of 20 kV and upwards, preferably higherthan 36 kV, is for a transformer to be inserted between the generatorand the power network, it is already known to attempt to eliminate thetransformer and generate the high voltage directly out to the powernetwork at its voltage level. Such generators are described, forinstance, in U.S. Pat. Nos. 4,429,244, 4,164,672 and 3,743,867.

However, the machine designs according to the above publications do notpermit optimal utilization of the electromagnetic material in thestator.

DESCRIPTION OF THE INVENTION

The object of the invention is thus to provide an electric generatorwhich can be used in a hydro-generator plant for such high voltage thatthe above-mentioned Δ/Y-connected step-up transformer can be omitted,i.e. a plant in which the electric generators are intended forconsiderably high voltages than conventional machines of correspondingtype, in order to be able to execute direct connection to power networksat all types of high voltage.

This object has been achieved according to the invention in that a plantof the type described in the preamble to claim 1 is given the specialfeatures defined in the characterizing part of this claim, in that agenerator of the type described in the preamble to claim 34 is given thespecial features defined in the characterizing part of this claim, andin that a procedure of the type described in the preamble to claim 33includes the special measures defined in the characterizing parts ofrespective claims.

Thanks to the solid insulation in combination with the other featuresdefined, the network can be supplied without the use of an intermediatestep-up transformer even at network voltages considerably in excess of36 kV.

The fact that the solid insulation enables the windings to be arrangedfor direct connection to the high-voltage network, thus eliminating thestep-up transformer, offers great advantages over known technology.

The elimination of the transformer per se entails great savings, forinstance, and the absence of the transformer also results in severalother simplifications and thus savings.

A plant of this type is often arranged in a rock chamber where, withconventional technology, the transformer is arranged either in directconnection with the generator in the rock chamber or above ground at adistance of several hundred meters and connected to the generator by abusbar system. Compared with the first alternative, elimination of thetransformer enables the volume of the rock chamber to be greatlyreduced. The fire risk entailed with an oil-insulated transformer isalso eliminated therefore reducing the necessity for extensivefire-safety precautions such as special evacuation routes for personnel.

In the alternative in which the transformer is placed above ground thebusbar system is more extended due to the longer distance between thegenerator and the transformer. Since the current in the busbars(normally with aluminium conductors) is considerable, in the order of10-20 kA, the power losses are large. Moreover, busbar systems introducea risk for 2- and 3-phase faults during which the currents areconsiderable.

With the present invention two major objectives are achieved:

-   -   The losses in the busrun are reduced due to the high voltage.    -   The risk for 2- and 3-phase failures is considerably reduced due        to the use of insulated HV cables.

The reduction in the number of electrical components achieved with theinvention therefore means that the corresponding safety equipment can beomitted.

Furthermore, the rock chamber need not be blasted to allow laying of thebusbar system, which entails a saving in rock chamber space of severalthousand cubic meters.

The plant according to the invention also enables several connectionswith different voltage levels to be arranged, i.e. the invention can beused for all auxiliary power in the power station.

In all, the advantages mentioned above entail radically improved totaleconomy for the plant. The plant cost, typically in the order of somehundred million SEK, is reduced by 30-50%. Operating economy is improvedboth by less need for maintenance and by an increase in the degree ofefficiency by 1-1.5%. For an operating time of 8000 h/year, an outputlevel corresponding to 150 MVA, a kWh price of SEK 0.20 and a usefulservice life of 30 years the gain would be approximately SEK 75-100million per generator.

In a particularly preferred embodiment of the plant and generatorrespectively, the solid insulation system comprises at least two layers,each layer constituting essentially an equipotential surface, and alsointermediate solid insulation therebetween, at least one of the layershaving substantially the same coefficient of thermal expansion as thesolid insulation.

This embodiment constitutes an expedient embodiment of the solidinsulation that in an optimal manner enables the windings to be directlyconnected to the high-voltage network and where harmonization of thecoefficients of thermal expansion eliminates the risk of defects, cracksor the like upon thermal movement in the winding.

It should be evident that the windings and the insulating layers areflexible so that they can be bent.

It should also be pointed out that the plant according to the inventioncan be constructed using either horizontal or vertical generators, whichmay be of either underground or aboveground type.

The above and other preferred embodiments of the invention are definedin the dependent claims.

The major and essential difference between known technology and theembodiment according to the invention is thus that this is achieved witha magnetic circuit included in an electric generator which is arrangedto be directly connected via only breakers and isolators to a highsupply voltage in the vicinity of between 20 and 800 kV, preferablyhigher than 36 kV. The magnetic circuit thus comprises a laminated corehaving at least one winding consisting of a threaded cable with one ormore permanently insulated conductors having a semiconducting layer bothat the conductor and outside the insulation, the outer semiconductinglayer being connected to earth potential.

To solve the problems arising with direct connection of electricmachines to all types of high-voltage power networks, the generator inthe plant according to the invention has a number of features asmentioned above, which differ distinctly from known technology.Additional features and further embodiments are defined in the dependentclaims and are discussed in the following.

Such features mentioned above and other essential characteristics of thegenerator and thus of the hydro-generator plant according to theinvention include the following:

-   -   The winding of the magnetic circuit is produced from a cable        having one or more permanently insulated conductors with a        semiconducting layer at both conductor and sheath. Some typical        conductors of this type are PEX cable or a cable with EP rubber        insulation which, however, for the present purpose are further        developed both as regards the strands in the conductor and the        nature of the outer sheath.    -   Cables with circular cross section are preferred, but cables        with some other cross section may be used in order to obtain        better packing density, for instance.    -   Such a cable allows the laminated core to be designed according        to the invention in a new and optimal way as regards slots and        teeth.    -   The winding is preferably manufactured with insulation in steps        for best utilization of the laminated core.    -   The winding is preferably manufactured as a multi-layered,        concentric cable winding, thus enabling the number of coil-end        intersections to be reduced.    -   The slot design is suited to the cross section of the winding        cable so that the slots are in the form of a number of        cylindrical openings running axially and/or radially outside        each other and having an open waist running between the layers        of the stator winding.    -   The design of the slots is adjusted to the relevant cable cross        section and to the stepped insulation of the winding. The        stepped insulation allows the magnetic core to have        substantially constant tooth width, irrespective of the radial        extension.    -   The above-mentioned further development as regards the strands        entails the winding conductors consisting of a number of        impacted strata/layers, i.e. insulated strands that from the        point of view of an electric machine, are not necessarily        correctly transposed, uninsulated and/or insulated from each        other.    -   The above-mentioned further development as regards the outer        sheath entails that at suitable points along the length of the        conductor, the outer sheath is cut off, each cut partial length        being connected directly to earth potential.

The use of a cable of the type described above allows the entire lengthof the outer sheath of the winding, as well as other parts of the plant,to be kept at earth potential. An important advantage is that theelectric field is close to zero within the coil-end region outside theouter semiconducting layer. With earth potential on the outer sheath theelectric field need not be controlled. This means that no fieldconcentrations will occur either in the core, in the coil-end regions orin the transition between them.

The mixture of insulated and/or uninsulated impacted strands, ortransposed strands, results in low stray losses.

The cable for high voltage used in the magnetic circuit winding isconstructed of an inner core/conductor with a plurality of strands, atleast two semiconducting layers, the innermost being surrounded by aninsulating layer, which is in turn surrounded by an outer semiconductinglayer having an outer diameter in the order of 20-200 mm and a conductorarea in the order of 40-3000 mm².

The solid insulation in a generator according to the invention alsooffers great advantages when constructing a hydro-generator plant. Theabsence of wet insulation means that the stator of the generator neednot be completed at the factory but can instead be delivered in partsand assembled on site. A stator of the size under consideration here islarge and heavy which has entailed transport problems with conventionaldesigns where the roads must be reinforced and dimensioned for the vastweight. This problem is eliminated since the stator for a generator canbe delivered in parts.

The invention thus also relates to the procedures as defined in claims30 and 33, where this possibility is exploited when building ahydro-generator plant and manufacturing a generator, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following detaileddescription of a preferred embodiment of constructing the magneticcircuit of the electric generator in the hydro-generator plant, withreference to the accompanying drawings in which

FIG. 1 shows a schematic axial end view of a sector of the stator in anelectric generator in the hydro-generator plant according to theinvention,

FIG. 2 shows an end view, partially stripped, of a cable used in thewinding of the stator according to FIG. 1,

FIG. 3 shows a simplified view, partially in section, of ahydro-generator arrangement according to the invention,

FIG. 4 shows a circuit diagram for the hydro-generator plant accordingto the invention,

FIG. 5 shows a section through a conventional hydro-generator plant.

FIG. 6 is a diagram showing a traditional solution for auxiliary powerfor a hydro plant, and

FIG. 7 is a diagram showing generators with build-in windings forgeneration of auxiliary power according to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

In order to understand certain aspects of the advantages of theinvention, reference is made initially to FIG. 5 showing an example of aconventional hydro-generator plant. This is of a type with thetransformer hall 501 situated some way from the generator hall 502, thelatter being in the form of a rock chamber housing the generator 503.The generator 503 is connected to the transformer in the transformerhall 501 via a busbar system 505 arranged in a tunnel system 504 severalhundred meters long. A plant according to the invention entirelyeliminates the part to the right of the line A-A in FIG. 5, whilesubstantially the same dimensions are retained in the generator hall502. A conventional plant without the transformer situated above groundas shown in FIG. 5 would instead require a considerably larger generatorhall 502 to allow space for the transformer and its auxiliary and safetyequipment.

The rotor 2 of the generator is also indicated in the schematic axialview through a sector of the stator 1 according to FIG. 1, pertaining tothe generator 100 (FIG. 3) included in the hydro-generator plant. Thestator 1 is composed in conventional manner of a laminated core. FIG. 1shows a sector of the generator corresponding to one pole pitch. From ayoke part 3 of the core situated radially outermost, a number of teeth 4extend radially in towards the rotor 2 and are separated by slots 5 inwhich the stator winding is arranged. Cables 6 forming this statorwinding, are high-voltage cables which may be of substantially the sametype as those used for power distribution, i.e. PEX cables.PEX=crosslinked polyethylene (XLPE). One difference is that the outer,mechanically-protective sheath, and the metal screen normallysurrounding such power distribution cables are eliminated so that thecable for the present application comprises only the conductor and atleast one semiconducting layer on each side of an insulating layer.Thus, the semiconducting layer which is sensitive to mechanical damagelies naked on the surface of the cable.

The cables 6 are illustrated schematically in FIG. 1, only theconducting central part of each cable part or coil side being drawn in.As can be seen, each slot 5 has varying cross section with alternatingwide parts 7 and narrow parts 8. The wide parts 7 are substantiallycircular and surround the cabling, the waist parts between these formingnarrow parts 8. The waist parts serve to radially fix the position ofeach cable. The cross section of the slot 5 also narrows radiallyinwards. This is because the voltage on the cable parts is lower thecloser to the radially inner part of the stator 1 they are situated.Slimmer cabling can therefore be used there, whereas coarser cabling isnecessary further out. In the example illustrated cables of threedifferent dimensions are used, arranged in three correspondinglydimensioned sections 51, 52, 53 of slots 5. An auxiliary power winding 9is arranged furthest out in the slot 5.

FIG. 2 shows a step-wise stripped end view of a high-voltage cable foruse in an electric machine according to the present invention. Thehigh-voltage cable 6 comprises one or more conductors 31, each of whichcomprises a number of strands 36 which together give a circular crosssection of copper (Cu), for instance. These conductors 31 are arrangedin the middle of the high-voltage cable 6 and in the shown embodimenteach is surrounded by a part insulation 35. However, it is feasible forthe part insulation 35 to be omitted on one of the conductors 31. In thepresent embodiment of the invention the conductors 31 are togethersurrounded by a first semiconducting layer 32. Around this firstsemiconducting layer 32 is an insulating layer 33, e.g. PEX insulation,which is in turn surrounded by a second semiconducting layer 34. Thusthe concept “high-voltage cable” in this application need not includeany metallic screen or outer sheath of the type that normal surroundssuch a cable for power distribution.

A hydro-generator with a magnetic circuit of the type described above isshown in FIG. 3 where the generator 100 is driven by a water turbine 102via a common shaft 101.

The stator 1 of the generator 100 thus carries the stator windings 10which are built up of the cable 6 described above. The cable 6 isunscreened and changes to a screened cable 11 at the cable splicing 9.

With a hydro-generator 100 according to the invention it is thuspossible to generate extremely high electric voltages of up toapproximately 800 kV. It is thus possible to electrically connect thehydro-generator 100 directly to a distribution or transmission network110 with an intermediate step-up transformer or similar electric machineas is generally the case in conventional plants where equivalentgenerators are able at most to generate voltages of up of 25-30 kV.

FIG. 4 illustrates a hydro-generator plant according to the presentinvention. In conventional manner, the generator 100 has an excitationwinding 112 and one (or more) auxiliary power winding(s) 113. In theshown embodiment of the plant according to the invention the generator100 is earthed via an impedance 103.

It can also be seen from FIG. 4 that the generator 100 is electricallyconnected via the cable splicing 9 to the screened cable 11 (see alsoFIG. 3). The cable 11 is provided with current transformers 104 inconventional manner, and terminates at 105. After this point 105 theelectric plant in the shown embodiment continues with busbars 106 havingbranches with voltage transformers 107 and surge arresters 108. However,the main electric supply takes place via the busbars 106 directly to thedistribution or transmission network 110 via isolator 109 andcircuit-breaker 111.

A hydro-generator plant according to the invention is designed foroperation either to generate electric voltage for the power network asdescribed above, or as a pump plant, i.e. to be driven from the electricpower network 110. The generator 100 then operates as a motor to drivethe turbine 102 as a pump.

Thus, with the hydro-generator 100, no intermediate coupling of astep-up transformer is required. With the hydro-generator plantaccording to the present invention, therefore, several transformer andbreaker units previously necessary are eliminated, which is obviously anadvantage—not least from the aspects of cost and operating reliability.

Although the hydro-generator and the plant in which this generator isincluded have been described and illustrated in connection with anembodiment by way of example, it should be obvious to one skilled inthat art that several modifications are possible without departing fromthe inventive concept. The generator may be earthed directly, forinstance, without any impedance. The auxiliary windings can be omitted,as also other components shown. Although the invention has beenexemplified with a three-phase plant, the number of phases may be moreor less.

1. A hydrogenerator plant for connection to a high voltage transmissionor distribution network comprising: at least one rotating electricmachine for high voltage coupled to a turbine via shaft means, saidelectric machine comprising at least one winding formed of a conductorincluding a plurality of insulated conductive elements, and at least oneuninsulated conductive element; a covering surrounding the conductorincluding an inner layer having semiconducting properties, a solidinsulating layer surrounding the inner layer and an outer layer havingsemiconducting properties surrounding the insulating layer, said innerlayer being in contact with the uninsulated element such that the innerlayer has the same potential as the conductor, and said at least onewinding being directly connectable to the transmission or distributionnetwork, the voltages being across a range of transmission ordistribution voltages.
 2. The plant as claimed in claim 1 wherein the atleast two semiconducting layers each form essentially an equipotentialsurface, and wherein at least one of the layers has substantially thesame coefficient of thermal expansion as the solid insulation.
 3. Theplant as claimed in claim 1, wherein the generator comprises a magneticcircuit with a magnetic core.
 4. The plant as claimed in claim 3,wherein the electric machine includes a core comprising laminated sheetof at least one of cast iron, powder-based iron, and rough forge iron.5. The plant as claimed in claim 1 wherein the winding comprises acable.
 6. The plant as claimed in claim 5, wherein at least two of saidlayers have substantially the same coefficient of thermal expansion. 7.The plant as claimed in claim 1, wherein the inner semiconducting layeris at substantially the same potential as the conductors.
 8. The plantas claimed in claim 1, wherein the outer semiconducting layer forms anequipotential surface surrounding the conductors.
 9. The plant asclaimed in claim 8, wherein said outer semiconducting layer is connectedto a predefined potential.
 10. The plant as claimed in claim 9, whereinthe predefined potential is earth potential.
 11. A plant as claimed inclaim 9 wherein the coils in the stator are distributed and have a coilspan different from the pole pitch.
 12. The plant as claimed in claim 1,wherein the cable also comprises a metal screen and a sheath.
 13. Theplant as claimed in claim 1 including a stator cooled at earth potentialby means of a fluid.
 14. The plant as claimed in claim 1 wherein theouter semiconducting layer is connected to earth potential.
 15. Theplant as claimed in claim 1, wherein the electric machine includes arotor inductively connected to the high voltage.
 16. The plant asclaimed in claim 15, wherein the rotor is cylindrical in shape, hassalient poles and also has a constant air gap.
 17. The plant as claimedin claim 16, wherein the electric machine includes a stator having astator winding formed as at least one of an integral slot winding, and afractional slot winding.
 18. The plant as claimed in claim 17, whereinthe stator has a pole pitch and the winding is distributed and includesa coil having a coil span different from the pole pitch.
 19. The plantas claimed in claim 1, wherein the cable has a conductor area of aboutbetween 40 and 3000 mm² and an outer cable diameter of about between 20and 250 mm.
 20. The plant as claimed in claim 19, wherein the cable iscooled by gas or liquid inside current-carrying conductors.
 21. Theplant as claimed in claim 1, wherein the electric machine is designedfor high voltage and arranged to supply the out-going electric networkdirectly without any intermediate connection of a transformer.
 22. Theplant as claimed in claim 21, wherein at least one electric machine isearthed via an impedance.
 23. The plant as claimed in claim 21, whereinelectric machine is directly earthed.
 24. The plant as claimed in claim21, wherein said plant is operative as at least one of a pump andturbine station, the electric machine being arranged to function as atleast one of a motor driven directly from the transmission ordistribution network and as a generator, generating voltage for thetransmission or distribution network.
 25. The plant as claimed claim 21,wherein the electric machine is arranged to generate power to variousvoltage levels.
 26. The plant as claimed in claim 25, wherein at leastone electric machine includes a separate auxiliary winding for producingauxiliary power at one of said voltage levels.
 27. The plant as claimedin claim 1, comprising a plurality of electric machines, each of whichlacks an individual step-up transformer, but which, via a systemtransformer common to the electric machines, is connected to thetransmission or distribution network.
 28. The plant as claimed in claim1, including a common earth system.
 29. The plant as claimed in claim 1,wherein the winding of the electric machine is operable forself-regulating field control and lacks auxiliary means for control ofthe field.
 30. The plant as claimed in claim 1, wherein the electricmachine includes a stator comprising a plurality of stator limitationshaving openings for receiving the winding, said laminations beingassembled into a stack with the openings aligned, and the windingcomprises a cable threaded into the openings or the stacking laminationsof the stator at the manufacturing facility or at the generation plantsite.
 31. An electric generator for a high voltage included in ahydro-generator plant in which the generator is coupled to a turbine viashaft means, said generator comprising at least one winding including aconductor, a solid insulation covering including an inner layer havingsemiconducting properties; a solid insulating layer surrounding theinner layer and an outer layer having semiconducting propertiessurrounding the insulation layer; said conductor formed of a pluralityof conductive elements including at least one uninsulated element incontact with the inner layer and a plurality of insulated elements; andwherein each winding is directly connectable to a high voltagetransmission or distribution network, and the inner layer forms anequipotential surface about the conductor.
 32. A hydrogenerator plantincluding a rotating high voltage electric machine comprising a stator;a rotor and a winding, wherein said winding comprises a cable includinga current-carrying conductor and a magnetically permeable, electricfield confining cover surrounding the conductor, the cover including aninner layer having semiconducting properties, a solid insulationsurrounding the inner layer and an outer layer having semiconductingproperties surrounding the solid insulation, said cable forming at leastone uninterrupted turn in the corresponding winding of said machine, andwherein the conductor includes a plurality of insulated conductivestrands and at least one uninsulated electrically conductive strand incontact with the inner layer, such that said conductor and inner layerare at the same potential.
 33. The hydrogenerator plant of claim 32,wherein the outer layer has a conductivity sufficient to establish anequipotential surface around the conductor.
 34. The hydrogenerator plantof claim 32, wherein the cover is formed of a plurality of integrallybonded layers, and wherein said plurality of layers are substantiallyvoid free.
 35. A hydrogenerator plant for direct connection to a highvoltage transmission or distribution network comprising: at least onerotating electric machine for high voltage coupled to a turbine viashaft means, said electric machine including at least one windingcomprising a conductor and a magnetically permeable, electric fieldconfining insulating covering surrounding the conductor including aninner layer having semiconducting properties, a solid insulationsurrounding the inner layer and an outer layer having semiconductingproperties surrounding the insulating layer, said conductor including atleast one of a plurality of insulated conductive elements, and at leastone uninsulated conductive element being in contact with the inner layersuch that said conductor and inner layer are at the same potential; andsaid at least one winding being directly connectable to the transmissionor distribution network.